1. Field of the Invention
The present invention relates to novel organic polyisocyanates having predominantly or entirely blocked isocyanate groups, in which at least two different blocking agents are present, to a process for their production and to their use as crosslinking agents in heat curable, polyurethane coating compositions.
2. Description of the Prior Art
Blocking organic polyisocyanates with blocking agents and the use of the blocked polyisocyanates as a crosslinking resin for polyurethane baking lacquers are known. Thus, for example, S. Petersen in Liebigs Ann. Chem. 562 (1949), p. 205 et seq mentions the possibility of blocking 1,6-diisocyanatohexane with blocking agents such as malonic acid diethyl ester, acetoacetic acid diethyl ester or piperidine. However, the resultant reaction products display a marked tendency to crystallize and for this reason alone are unusable as crosslinking resins for baking lacquers dissolved in lacquer solvents.
DE-OS 2,436,872 describes liquid, blocked polyisocyanates, which are prepared from isophorone diisocyanate and malonic acid esters. Analogous liquid polyisocyanates are also described in DE-OS 3,001,060. These are malonic or acetoacetic ester blocked trimers of isophorone diisocyanate with isocyanurate groups. The use of such polyisocyanates which are entirely blocked with CH-acid esters as crosslinking resins for polyhydroxyl compounds results, after crosslinking, in an amide-ester compound instead of urethane groups as is known from Wicks, Kostyk J. Coat. Techn. 49, 1977, p. 77. This is also apparent from slightly poorer properties of the resulting coatings and, in particular, in reduced storage stability of the coating compositions (c.f. examples 10 and 11).
EP-A-0,096,210 describes one-component binders wherein the isocyanate component is blocked entirely with secondary amines, for example diisopropylamine. Based on the disclosure, these blocked polyisocyanates are primarily useful as solid products for powder coating applications.
EP-A-0,125,438 also describes one-component binders wherein the isocyanate component is blocked with secondary amines. Since, as may be seen from the examples, the intermediately produced isocyanate components are formulated into the ready-to-use lacquers, it is not apparent to what extent these isocyanate components are stable in terms of crystallization.
DE-OS 2,812,252 describes polyisocyanate crosslinking agents blocked with 1,2,4-triazole. Again, the emphasis is on the production of powder coatings. Polyisocyanates blocked with 1,2,4-triazole generally have a marked tendency to crystallize.
EP-A-0,403,044 describes blocked polyisocyanates with at least two blocking agents wherein the unblocking temperatures are at least 40.degree. C. apart. The intended objective is step-wise crosslinking. The disclosed blocking agents include .epsilon.-caprolactam, oximes and butanol, which crosslink at significantly above 150.degree. C.
The industrially and economically important polyisocyanates having isocyanurate groups and prepared from 1,6-diisocyanatohexane have to date remained without commercial importance when blocked with blocking agents which are released at low temperatures such as malonic esters or diisopropylamine. This is because, when dissolved in lacquer solvents, such blocked polyisocyanates are not stable in storage (see also comparative examples 1 and 2).
An object of the present invention is to provide novel, blocked lacquer polyisocyanates which may be used for the production of baking lacquers which have a maximum baking temperature of 140.degree. C. and which may be dissolved in known lacquer solvents to produce storage stable solutions.
This object may be achieved with the polyisocyanates according to the invention which are described in greater detail below.